High stability thin film alloy resistors

ABSTRACT

High stability thin film alloy resistors can be made from a constant conductivity alloy comprising a predominant amount of platinum, rhodium, iridium, palladium, gold, or silver and a minor amount of at least two metals selected from tungsten, rhenium, tantalum, molybdenum, hafnium, zirconium, or niobium. The constant conductivity alloy is deposited onto a nonconducting substrate to a thickness of between 10 and 5,000 angstroms.

United States Patent 1191 Ang et a1. Sept. 3, 1974 [54] HIGH STABILITY THIN FILM ALLOY 3,472,691 10/1969 Kooy et a1. 117 107 RESISTORS 3,617,373 11 1971 Mott 117 107 3,627,577 12/1971 Ste1de1 117/107 [75] Inventors: Choh-Yi Ang, Santa Ana; R rt F- R27,287 2/1972 Lepselter 117/212 Bailey, Los Alamitos; John R. Ogre!" Hawthorne of Cahf Primary Examiner-Cameron K. Weiffenbach [73] Assignee: TRW Inc., Redondo Beach, Calif. Attorney, 8 3 Firm-Daniel Anderson; Alan D.Ak ;R rtM.D 'd 22 Filed: Dec. 30, 1971 0 e High stability thin film alloy resistors can be made [52] US. Cl 117/227, 29/620, 12107510972, from a constam conductivity alloy comprising a dominant amount of platinum, rhodium, iridium, pal- {51] p B44! 1/18 Hole 7/00 Holc 17/09 ladium, gold, or silver and a minor amount of at least [58] Flew of Search 117/227 two metals selected from tungsten, rhenium, tantalum, 204/192 29/620 molybdenum, hafnium, zirconium, or niobium. The constant conductivity alloy is deposited onto a non- [56] References cued conducting substrate to a thickness of between 10 and UNITED STATES PATENTS 5,000 angstroms, 3,180,751 4/1965 Law 117/107 3,463,636 8/1969 Ogren 75/165 10 Clams 2 Drawmg Flgul'es Fig. I

PATEIIIE sEP 3mm 3, 3, sum 10? 2 I L I I I III I000 IOOOO ROOM TEMPERATURE I I II I II II IIIIII- IOO IIIIIII I IIIIIII I IIIIIIII I I I IIIII SHEET RESISTANCE IN Q/I: RESISTIVE PROPERTIES OF Pt-W-Re THIN FILMS IOO 2 NI SSBNXOIHJ.

Choh-Yi Ang Robert F Bailey J 0 h n R. 09 re n INVENTORS I BACKGROUND OF THE INVENTION Previously, thin film resistors have been made from tantalum, refractory metal nitrides, and nickelchromium alloys. Although each of the prior art thin film materials have been satisfactory for most microelectronic purposes, advanced applications require properties which these materials cannot meet. For example, microelectronic circuits today require ahigher degree of stability and reliability in a greater number of environmental situations. The chromium, chromium alloys, tantalum, and refractory metal alloys currently in use are susceptible to corrosion by reaction with environmental gases and residual solvents from device processing. Accordingly, these materials, especially when in the form of films, are chemically modified in time which leads to changesin the electrical properties. Some priorart materials do possess high corrosion, but their resistivities are usually too low for many applications. One example, of these materials is the 98% gold-2% chromium alloy which has a resistivity of only 30 micro ohm-cm. Two of the frequently used prior art materials are 80% nickel-20% chromium and reactively sputtered tantalum. In thin film form, these materials have sheet resistances of 200 ohms per square, but with thermal coefficients of resistivity from minus 300 to plus 200 ppm/C over the 25C to 125C temperature range. Future microelectronic applications will require much lower temperature coefficients of resistivity than those of nichrome and sputtered tantalum, in addition to the high stability providedby extreme corrosion resistance. I

U.S. Pat. No. 3,463,636 discloses a bulk form of a constant conductivity alloy of similar composition as disclosed herein. The distinguishing feature, however, is that when films of less than 5,000 angstroms are sputteredonto a substrate in a circuit pattern, outstanding and unique resistor properties may be obtained. For example, the resistivity of a film of the present alloy may be several times greater than the resistivity of the bulk alloy which is about 172 to IQOuO-cm. In addition to this property difference, the temperature coefficient of resistivity exhibited by the film is smaller than that exhibited by the bulk form. Temperature coefficients of resistivity for the films of this invention exhibit 150 ppm/"C in a temperature range of 25C to +l25C whereas the bulk material exhibits about +72 to +367 ppm/"C in the same temperature range.

SUMMARY OF THE INVENTION The present invention is related generally to a thin film resistor comprising a predominant amount of a noble metal and a minor amount of at least two refractory metals which have been deposited on a substrate to a thickness of less than 5,000 angstroms, generally, to 500 angstroms. Thin film circuits are produced preferably by sputtering a compacted blend of metal powders onto a dielectric substrate. Formation of the circuit pattern is most readily achieved by backsputtering the deposited metal, although photolithographic masks and chemical etches have been used also. Thin film resistors produced in either manner exhibit low and controllabletemperature coefficients of resistivity, long term electrical stability in environmentel-high stress conditions, and high power density capa- "bilities. p

DESCRIPTION OF THE PREFERRED EMBODIMENTS BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 shows the increasing sheet resistance in ohms per square as it relates to the decreasing thickness in angstroms at room temperature; and

FIG. 2 depicts the range of temperature coefficients of resistivity in parts per million per degree Centigrade of 23 readings at various film thickness in angstroms.

The graphical representations in FIG. 1 and FIG. 2 more clearly show the trends represented by the values in the Table. FIG. 1 shows good consistency with the rule of thin film conduction, i.e., the thinner the film, the higher the sheet resistance.

FIG. 2 shows the clustering of thermal coefficient of resistivity points which were obtained from the last column in the Table. Seventeen of the twenty-three data points which were obtained fall within the range :50 ppm/C, with the smallest value being 0.

The ternary alloys used in this invention are comprised of a major portion of a noble metal selected from the group consisting of platinum, rhodium, iridium, palladium, gold, or silver which is alloyed with a minor portion of two refractory metals selected from tungsten, rhenium, tantalum, molybdenum, hafnium, zirconium, and niobium. These metals are formulated in proportions of 50% to 90% by weight of the noble metal with 50% to 70% by weight being preferred, and 10% to 50% by weight of two or more of the refractory metals and 30% to 50% by weight being preferred. Sputtering targets may be fabricated from a blend of constituent powders which have been isostaticallycompacted, or from an alloy prepared by melting the constituent metals. Although sintering of the powder metal compact is preferable, it is not critical to the success of the sputtering operation, and likewise if alloys specimens are used, remelting to improve homogeneity is preferable but not critical.

Substrates used in this invention may be selected from substantially any of the dielectric materials which can withstand the temperatures encountered in a sputtering operation. Specifically, materials which are suitable for use as substrates may be selected from silicon, ceramic, quartz, or glass.

Thereare several methods of producing metallic films in accordance with this invention. Vapor deposition, vacuum evaporation, sputtering, and screen printing are a few specific examples of the various methods which may be employed. Selection of the most suitable method will depend upon the metallic system involved, the thickness of the films required, and the types of product application.

A more complete understanding of the present invenl tion may be obtained by reference to the following example.

EXAMPLE A mixture containing seven grams of powdered platitrim and 3 grams of powdered tungsten 25% rhenium alloy were blended and isostatically compacted at 5 60,000 psi into a 3% inch diameter by 1/32 inch thick target. A specimen substrate of alumina-boro-silicate glass, 1.75 inch by 1.25 inch, and the alloy target were placed in a'MRC sputtering chamber and pumped to less than X torr. The target was sputter-cleaned first for l/2 hour at 100 watts at 10 microns pressure of argon. Maintaining RF power, the substrates were rotated to the sputtering position approximately 1.25

4 3. A thin film resistor according to claim I wherein; the alloy film is between 10 and 5,000 Angstroms in thickness.

palladium, gold, and silver and (ii) from 10% to 50% by weight of at least two metals selected from the group "consisting of tungsten, rhenium, tanta lum, molybdenum, hafnium, zirconium, and niobium, on a 1b. substrate selected from the group consisting of sili con, ceramic, glass and quartz. 2. A thin film resistor accordingto claim 1 wherein:

the constant conductivity alloy consists essentially of (i) from 50%,to 70% by weight of a metal selected from the group consisting of platinum, rhodium, iridium, palladium, gold, and silver and (ii) from to by weight of at least two metals selected from the group consisting of tungsten, rhenium, tantalum, molybdenum, hafnium, zirconium, and niobium.

inches from the target. A film of 154 angstroms thick- 5 g ness having a composition of 69.5% by weight platiposltlonmg a nonflollducitmg sj-lbstrate and a metal num, 2 by weight tungsten, and 37% by weight alloy source consistmg essentlally of from 50% to rhenium was deposited on the substrate. The resultant Weight m al Selected from the group thin film pattern was determined to have an initial resisfm of Plallnum, Thodlum, mdlum pallatance of 3,687 ohms at room temperature and after ex- 10 i 8 and Sliver and from 10% to 50% by posure to 125C for 236 hours in air, the resistance at we'ght of a least two metals Selected from the room temperature was determined to be 3,692 ohms, group cnslstmg f tungsten. rhemum l l" or a change of+0. 10%. The power handling capability E g zlrcomum and moblum of this film was determinedto be approximately 100 b an em 3. g d b k h watts per square inch, in an essentially non-dissipating l5 'g gs mg Sal c am er an ac 1 mg m di m.

c. depositing said metal alloy to a depth of less than Several additional specimens were made and their 5,000 Angstroms. resistance properties determined. The following table 5. A method according to claim 4 wherein:

- O I I 20 said metal alloy is deposited to a depth of 10 to 5,000

- M TABLE I Sheet 1 Thickness Composition 7: Resistance Resistance TRC in ppmlC Film N6. in A Pt-W -Re n/|:| Kn 25-125c 6 32 69.0-20.7-103 2l00 250 Not measured 7 83 I 69.6-l8.3-12.l .757. 42.78 +124 41.03 +90 58.82 +104 9 .154 69.5-21.8-8.7 162 6.31 +14 v 5.06 1 +16 8 282 678-206-116 140 9.87 +39 5.11 -21 5.44 15' 12 505 69.6-l8.5-ll.9 45 10.96 l8 "We claim: 45 an'g'strom.

1. A thin film resistor comprising: 6. A method according to claim 4 wherein; a. a film of less than 5,000 Angstroms of a constant said alloy is sputter-cleaned prior to sputter depositconductivity alloy consisting essentially of (i) from ing onto said substrate. 50% to %by weight of a metal selected from the 7. A method according to cla1m 4 mcludmg the fur- .group consisting of platinum, rhodium, iridium, ther stepof formmg a resistor pattern of said metal alloyby back sputtering. r 8. A method according to claim 4 including the further step of forming a resistor pattern of said metal alloy by chemical etching of a photolithographic pattern.

9. A method according to claim 4 wherein; said substrate is selected from the group consisting of silicon, ceramic, quartz, and glass. 1 '10. A method according to claim 9 wherein:

said metal alloy consists essentially of (i) 50% to 70% by weight of a metal selected from the group consisting of platinum, rhodium, iridium, palladium, gold, and silver and (ii) from 30% to 50% by weight of at least two metals selected from the group consisting of tungsten, rhenium, tantalum, molybdenum, hafnium, zirconium, and niobium.

4. A method for making'a thin film resistor compris- 

2. A thin film resistor according to claim 1 wherein: the constant conductivity alloy consists essentially of (i) from 50% to 70% by weight of a metal selected from the group consisting of platinum, rhodium, iridium, palladium, gold, and silver and (ii) from 30% to 50% by weight of at least two metals selected from the group consisting of tungsten, rhenium, tantalum, molybdenum, hafnium, zirconium, and niobium.
 3. A thin film resistor according to claim 1 wherein; the alloy film is between 10 and 5,000 Angstroms in thickness.
 4. A method for making a thin film resistor comprising: a. positioning a nonconducting substrate and a metal alloy source consisting essentially of from 50% to 90% by weight of a metal selected from the group consisting of platinum, rhodium, iridium, palladium, gold, and silver, and from 10% to 50% by weight of at least two metals selected from the group consisting of tungsten, rhenium, tantalum, molybdenum, hafnium, zirconium, and niobium, in an enclosed chamber, b. evacuating said chamber and back filling with argon, c. depositing said metal alloy to a depth of less than 5,000 Angstroms.
 5. A method according to claim 4 wherein: said metal alloy is deposited to a depth of 10 to 5,000 angstrom.
 6. A method according to claim 4 wherein; said alloy is sputter-cleaned prior to sputter depositing onto said substrate.
 7. A method according to claim 4 including the further step of forming a resistor pattern of said metal alloy by back sputtering.
 8. A method according to claim 4 including the further step of forming a resistor pattern of said metal alloy by chemical etching of a photolithographic pattern.
 9. A method according to claim 4 wherein; said substrate is selected from the group consisting of silicon, ceramic, quartz, and glass.
 10. A method according to claim 9 wherein: said metal alloy consists essentially of (i) 50% to 70% by weight of a metal selected from the group consisting of platinum, rhodium, iridium, palladium, gold, and silver and (ii) from 30% to 50% by weight of at least two metals selected from the group consisting of tungsten, rhenium, tantalum, molybdenum, hafnium, zirconium, and niobium. 